This invention relates to precision hydrodynamic bearings.
One important limitation to increasing track density of computer disk drives is spindle bearing performance. A disk drive whose spindle bearing has low runout can accommodate higher track densities which results in more data storage capacity per disk.
The kinematics of the spin axis of a spindle bearing determine the precision of the bearing. As the journal spins relative to the sleeve, the spin axis may trace out a path or orbit. The motion of this axis typically has components that are synchronous with the spin and repetitive in nature. These motions are termed repetitive runout. Other components of spin axis motion may be asynchronous and nonrepetitive with respect to spin. These components are termed nonrepetitive runout. As a general rule, spindle bearing precision is increased as repetitive and nonrepetitive runouts are decreased.
Ball bearing spindle systems make up the majority of prior art disk drives. The kinematics of the rolling elements in ball bearings result in relatively large nonrepetitive runout. This results from the fact that the lubricant film thicknesses in ball bearings are very thin providing little attenuation of geometric defects in the bearing. In addition, ball bearings produce forces on the disk drive structure to which it is attached which are of relatively high frequency and large amplitude.
Hydrodynamic spindle bearing designs are also known. The Hewlett-Packard Model No. 9154A, 3.5 inch micro-Winchester disk drive incorporates a hybrid hydrodynamic-ball bearing spindle. The performance of this bearing is degraded by the incorporation of the ball bearings. The Phillips video 2000 videocassette recorder utilizes a hydrodynamic bearing which employs grease as the lubricant limiting operation to low speeds. Other known hydrodynamic spindle bearings for disk drives employ a ferromagnetic fluid as the lubricant for the bearing. This fluid is retained or sealed in the bearing by magnetic fields set up in pole pieces at each end of the bearing. Unless the magnetic fields and clearances are very precisely matched at each end of the bearing, one seal will be stronger than the other and when the bearing heats up, the lubricant can be spilled. See U.S. Pat. No. 4,526,484.